The present invention relates to the general field of systems for injecting an air/fuel mixture into a turbomachine combustion chamber. It relates more particularly to an injection system provided with a cold plasma generator capable of controlling the reactivity of the air/fuel mixture during its injection into the combustion chamber.
The main objective of the conventional process of designing and optimizing a turbomachine combustion chamber is to reconcile the operational performance characteristics of the chamber (combustion efficiency, stability range, ignition and relight range, lifetime of the combustion region, etc.) according to the envisaged mission of the aircraft on which the turbomachine has been mounted, while minimizing the polluting emissions (nitrogen oxides, carbon monoxide, unburnt hydrocarbons, etc.). To do this, it is possible to vary in particular the nature and the performance characteristics of the system for injecting the air/fuel mixture into the combustion chamber, the distribution of the dilution air in the chamber and the dynamics of the air/fuel mixture in the chamber.
The combustion chamber of a turbomachine is typically composed of several systems, namely a system for injecting an air/fuel mixture into a flame tube, a cooling system and a dilution system. The combustion mainly takes place within a first part of the flame tube (primary zone) in which the flame is stabilized by means of air/fuel mixture recirculation zones induced by the air flow coming from the injection system. In this primary zone of the mixing tube, various physical phenomena occur, namely injection and atomization into fine droplets of the fuel, evaporation of the droplets, mixing of the fuel vapours with the air and chemical oxidation reactions in which the fuel is oxidized by the oxygen of the air. In the second part of the mixing tube (dilution zone), the chemical activity occurring is weaker and the flow is diluted by means of dilution holes.
To reduce the polluting emissions, especially nitrogen oxide emissions (of the NOx type), it is known to try to eliminate those zones of the flame tube where the temperature is above about 1800 K. To do this, it is necessary for the combustion flame to be in the presence of a rich or lean air/fuel mixture. For example, the air/fuel mixture of that zone of the flame tube where the chemical reactions take place may be made lean by increasing the flow rate of air assigned to the combustion. In this case, it thus helps in evaporating and mixing more and more fuel with the air before feeding the flame located in the combustion zone. The combustion flame therefore experiences a reduction in its richness.
However, increasing the air flow rate is not sufficient to completely eliminate the zones of stoichiometric mixing within the combustion region. In general, making the combustion leaner results in an increase in the vulnerability of the combustion region to extinction, so that the idling phases of the engine can no longer be obtained.
To solve this problem, engine designers have developed the concept called “staged combustion” which may take two forms, namely what are called “double-staged” combustion chambers and “multipoint” injection systems.
Double-staged combustion chambers are chambers in which the fuel injectors are distributed around what is called a “pilot” head and around what is called a “take-off” head. The pilot head operates permanently and thus prevents the combustion region from being extinguished, whereas the take-off head is designed to reduce NOx-type emissions. Also this solution appears satisfactory, a double-staged chamber is still difficult to control and is expensive owing to the doubling of the number of fuel injectors as compared with a conventional single-head combustion chamber.
“Multipoint” injection systems for injecting the air/fuel mixture are systems in which the injection of air and fuel takes place via several independent ducts and is regulated according to the operating speed of the turbomachine. The main drawback of such multipoint injection systems lies in the complexity of the various fuel circuits and of the regulating system.
Patent U.S. Pat. No. 6,453,660 teaches a multipoint injection system provided with a hot plasma generator. In that document, provision is made to equip the end of the main fuel injector with a hot plasma generating device. A high-energy discharge occurs in the fuel flow, thus allowing the fuel molecules to be ionized and partly dissociated. However, such an injection system is not completely satisfactory. Firstly, the multipoint architecture remains complex and difficult to control. Secondly, the high-energy discharge takes place only in the main fuel flow, which limits the effectiveness of such an injection system in combating the risk of extinction of the combustion region.